The Doppler shift is the observed change in frequency of a wave due to the relative velocity between an observer and the object from which the wave emanates. This change in frequency is directly and linearly dependent on the relative velocity. As a result, the Doppler shift is useful in many systems, e.g. a Doppler radar, to detect the speed of target objects.
A typical Doppler radar emits a periodic electromagnetic wave, for example at a frequency of 13.325 GHz, toward a target object. Some portion of the transmit signal is reflected, or "echoed," back to the radar where it is received. The relative velocity between the source and the target results in a Doppler shift in the frequency of the echoed signal. By measuring the frequency shift of the reflected wave, circuitry in a Doppler radar can determine how fast the target is moving relative to radar.
For example, Doppler radar devices used in meteorology can determine how a fast storm front is approaching by measuring the Doppler shift of a signal reflected off condensation in a rain cloud. Another application of Doppler radar includes Doppler navigation for determining the airspeed of a jet, i.e. the relative velocity between the aircraft and a fixed point on the ground.
Doppler shift detectors can also be used in sonar devices, which generate and emit a sound wave that is reflected off a target object. This sound wave too will undergo a frequency shift based on the relative velocity between the sonar device and the target object.
Since the Doppler shift can be a very small fraction of the transmitter frequency, conventional Doppler systems are generally incapable of measuring the frequencies of the transmit signal and the receive signal to a sufficient degree of precision to detect the Doppler shift. Therefore, conventional systems do not measure the frequency of the echoed wave directly, Instead, they measure the beat frequency shift of a signal derived from adding the echo signal to a reference signal generated from the transmitter.
Conventional techniques for detecting Doppler shifts, therefore, use analog circuitry or are otherwise difficult to implement on a monolithic semiconductor substrate of an integrated circuit, e.g., in CMOS technology.